Photovoltaic panel circuitry

ABSTRACT

Circuits integrated or integrable with a photovoltaic panel to provide built-in functionality to the photovoltaic panel including safety features such as arc detection and elimination, ground fault detection and elimination, reverse current protection, monitoring of the performance of the photovoltaic panel, transmission of the monitored parameters and theft prevention of the photovoltaic panel. The circuits may avoid power conversion, for instance DC/DC power conversion, may avoid performing maximum power tracking to include a minimum number of components and thereby increase overall reliability.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 16/458,402, filed on Jul. 1, 2019, which is a continuation of U.S.application Ser. No. 15/838,805, filed on Dec. 12, 2017, now U.S. Pat.No. 10,381,977, which is a continuation of U.S. application Ser. No.14/631,227, filed on Feb. 25, 2015, now U.S. Pat. No. 9,923,516, whichis a continuation application of U.S. application Ser. No. 13/753,041,filed on Jan. 29, 2013, now U.S. Pat. No. 8,988,838. This applicationclaims priority to United Kingdom Application GB1201506.1 filed Jan. 30,2012. Benefit of the filing date of these prior applications is herebyclaimed. The contents of all of these applications are herebyincorporated by reference in their entireties for all purposes.

BACKGROUND 1. Technical Field

Aspects of the present disclosure relate to distributed power systems,particularly a circuit for integrating with or attaching to aphotovoltaic panel.

2. Description of Related Art

A conventional photovoltaic distributed power harvesting system multiplephotovoltaic panels are interconnected and connected to an inverter.Various environmental and operational conditions impact the power outputof the photovoltaic panels. For example, the solar energy incident,ambient temperature and other factors impact the power extracted fromeach photovoltaic panel. Dependent on the number and type of panelsused, the extracted power may vary widely in the voltage and currentfrom panel to panel. Changes in temperature, solar irradiance andshading, either from near objects such as trees or far objects such asclouds, can cause power losses. Owners and even professional installersmay find it difficult to verify the correct operation of the system.With time, many more factors, such as aging, dust and dirt collectionand panel degradation affect the performance of the solar photovoltaicdistributed power system.

Data collected at the inverter may not be sufficient to provide propermonitoring of the operation of the system. Moreover, when the systemexperiences power loss, it is desirable to ascertain whether it is dueto environmental conditions or from malfunctions and/or poor maintenanceof the components of the solar power distributed power system.Furthermore, it is desirable to easily locate any particular solar panelthat may be responsible for power loss. However, information collectionfrom each panel requires a means of communication to a central datagathering system. It is desirable to control data transmission, to avoidtransmission collisions, and ascertain each sender of data. Such arequirement can be most easily accomplished using a duplex transmissionmethod. However, a duplex transmission method requires additionaltransmission lines and complicates the system. On the other hand,one-way transmission may be prone to collisions and makes it difficultto compare data transmitted from the various sources. Due to the widevariability of power output of such systems, and the wide range ofenvironmental conditions that affect the power output, the outputparameters from the overall system may not be sufficient to verifywhether the solar array is operating at peak power production. Localdisturbances, such as faulty installation, improper maintenance,reliability issues and obstructions might cause local power losses whichmay be difficult to detect from overall monitoring parameters.

Electric arcing can have detrimental effects on electric powerdistribution systems and electronic equipment. Arcing may occur inswitches, circuit breakers, relay contacts, fuses and poor cableterminations. When a circuit is switched off or a bad connection occursin a connector, an arc discharge may form across the contacts of theconnector. An arc discharge is an electrical breakdown of a gas whichproduces an ongoing plasma discharge, resulting from a current flowingthrough a medium such as air which is normally non-conducting. At thebeginning of a disconnection, the separation distance between the twocontacts is very small. As a result, the voltage across the air gapbetween the contacts produces a very large electrical field in terms ofvolts per millimeter. This large electrical field causes the ignition ofan electrical arc between the two sides of the disconnection. If acircuit has enough current and voltage to sustain an arc, the arc cancause damage to equipment such as melting of conductors, destruction ofinsulation, and fire. The zero crossing of alternating current (AC)power systems may cause an arc not to reignite. A direct current systemmay be more prone to arcing than AC systems because of the absence ofzero crossing in DC power systems.

In Photovoltaic Power Systems and The National Electrical Code,Suggested Practices: Article 690-18 requires that a mechanism beprovided to disable portions of the PV array or the entire PV array.Ground-fault detection, interruption, and array disablement devicesmight, depending on the particular design, accomplish the followingactions; sense ground-fault currents exceeding a specified value,interrupt or significantly reduce the fault currents, open the circuitbetween the array and the load, short the array or sub-array

According to the IEE wiring regulations (BS 7671:2008) a residualcurrent device (RCD) class II device on the direct current (DC)photovoltaic side for disconnection because of ground-fault current isreferred to in regulation 712.412.

The use of photovoltaic panel based power generation systems areattractive from an environmental point of view. However, the cost ofphotovoltaic panels and their relative ease of theft, might limit theiradoption for use in power generation systems.

Thus there is a need for and it would be advantageous to have circuitryintegrable or integrated with a photovoltaic panel which providesfeatures including: monitoring of the photovoltaic panel, ground-faultdetection and elimination, arc detection and elimination, theftprevention and a safety mode of operation while maintaining a minimalnumber of components in the circuit to decrease cost and increasereliability.

BRIEF SUMMARY

Various circuits are disclosed which are integrated or integrable with aphotovoltaic panel to provide built-in functionality to the photovoltaicpanel including safety features such as arc detection and elimination,ground fault detection and elimination, reverse current protection,monitoring of the performance of the photovoltaic panel, transmission ofthe monitored parameters and theft prevention of the photovoltaic panel.The circuits may avoid power conversion, for instance DC/DC powerconversion, may avoid performing maximum power tracking to include aminimum number of components and thereby increase overall reliability.

According to features of the present invention, there is provided acircuit for a photovoltaic panel. The circuit may include an inputterminal attachable to the photovoltaic panel, an output terminal and acontroller. A switch may be operatively connected between the inputterminal and the output terminal and a control terminal operativelyconnected to the controller. The switch when closed may provide a lowimpedance direct current path for direct current producible by thephotovoltaic panel to the output terminal. The circuit may includemultiple input terminals and multiple output terminals, high voltageinput and output terminals and low voltage input and output terminalswhich may or may not be at ground potential. The circuit may furtherinclude an output bypass circuit connectible across the outputterminals. The bypass circuit may be operable to bypass current aroundthe switch and around the photovoltaic panel. The circuit may avoidpower, voltage and current conversion between the input terminal and theoutput terminal. The circuit may further include at least one sensoroperatively attached to the controller. The sensor may be configured tomeasure at least one parameter such as current through the inputterminal, voltage at the input terminal, current through the outputterminal or voltage at the input terminal. A transmitter may beoperatively attached to the controller. The transmitter may be operableto transmit the at least one parameter. The circuit may further includea permanent attachment to the photovoltaic panel.

The circuit may include at least two modules or at least three modulesoperatively connected to or integrated with the controller selected froma theft detection module, an arc elimination module, a ground faultdetection module and/or a safety module. The theft detection module maybe operable to detect a potential theft of the photovoltaic panel byconfiguring the controller to activate the switch and to disconnect thephotovoltaic panel from the output terminal(s) responsive to thepotential theft detection.

The arc elimination module may be operable to detect an arc within or inthe vicinity of the photovoltaic panel or the circuit. The controllermay be configured to activate the switch and to disconnect thephotovoltaic panel from the output terminal responsive to a detection ofthe arc. The ground fault detection module may be operable to detect aground fault within the circuit or the photovoltaic panel. Thecontroller may be configured to activate the switch and to disconnectthe photovoltaic panel from the output terminal responsive to adetection of the ground fault. For the safety module, the controller maybe configured to activate the switch to select either a safe operatingmode to produce a safe limited output power on the output terminal or anormal operating mode to produce a substantially maximum output powerfrom the photovoltaic panel.

The circuit may further include a monitoring module operable to monitorthe performance of the photovoltaic panel. The monitoring module may beoperable to detect at least one condition of over current, over voltageor over temperature. The controller may be configured to activate theswitch responsive to the at least one condition.

According to features of the present invention, a circuit for aphotovoltaic panel is provided. The circuit includes input terminalsattachable to the photovoltaic panel, output terminals and a controller.A switch may be operatively connected between an input terminal and anoutput terminal. The switch may include a control terminal operativelyconnected to the controller. The switch may include a single pole switchwith a first pole connected to at least one of the input terminals, asecond pole connected to at least one of the output terminals and acontrol terminal operatively connected to the controller. The circuitmay further include an input bypass circuit connectible across the inputterminals. The bypass circuit is operable to bypass current around thephotovoltaic panel. The circuit may further include an output bypasscircuit connectible across the output terminals. The bypass circuit maybe operable to bypass current around the switch and around thephotovoltaic panel. The switch when closed may provide a low impedancepath for direct current between the photovoltaic panel to the outputterminal.

The circuit may avoid power conversion between the input terminal andthe output terminal. The circuit may also include a direct current (DC)to DC power converter to perform power conversion between the inputterminal and the output terminal. The power converter may be a buckcircuit, a boost circuit, a buck plus boost circuit, Cuk converter, or abuck-boost circuit.

The circuit may include at least two modules or at least three modulesmay be operatively connected or integrated with the controller includinga monitoring module, a theft detection module, an arc elimination moduleand/or a ground fault detection module. The monitoring module may beoperable to monitor the performance of the photovoltaic panel. Themonitoring module may be operable to detect at least one condition suchas over rated current, under rated current, over rated voltage, underrated voltage over rated temperature or under rated temperature. Thecontroller may be configured to activate the switch responsive to the atleast one condition. The monitoring module may be operable to monitorperformance of the circuit. The theft detection module may be operableto detect a potential theft of the photovoltaic panel. The controllermay be configured to activate the switch and to disconnect thephotovoltaic panel from the output terminal responsive to the potentialtheft detection. The arc elimination module may be operable to detectarcing within or in the vicinity of the photovoltaic panel. Thecontroller is configured to activate the switch and to disconnect thephotovoltaic panel from the output terminal responsive to an arcdetection. The ground fault detection module may be operable to detect aground fault within the junction box or in the vicinity of thephotovoltaic panel. The controller is configured to activate the switchand to disconnect the photovoltaic panel from the output terminalresponsive to a ground fault detection.

The circuit may further include a safety module operatively connected tothe controller. The controller may be configured to activate the switchto select either a safe operating mode to produce a safe working outputpower on the output terminal or a normal operating mode to produce asubstantially maximum output power.

According to features of the present invention, there is provided amethod performable in a photovoltaic solar power harvesting system. Themethod performs by a circuit integrated or integrable with aphotovoltaic panel to form a photovoltaic module. The circuit has inputterminals and output terminals. The circuit may include a controlleradapted to monitor in parallel multiple types of malfunctions. Thecontroller is adapted to control at least one switch connected betweenthe input terminals and the output terminals to activate the switch andto disconnect thereby the photovoltaic panel from at least one of theoutput terminals and to bypass the output terminals upon detecting atleast one of multiple malfunctions. The malfunctions monitored by thecontroller may include: an arc, a potential theft, a ground fault or amonitored parameter fault. The detection of the arc may be in thephotovoltaic module or in the vicinity of the photovoltaic module. Thedisconnection of the photovoltaic panel from the at least one outputterminal may be responsive to eliminate the arc. The potential theft ofthe photovoltaic module and the disconnection of the photovoltaic panelfrom the at least one output terminal may render the photovoltaic moduleinoperable outside the photovoltaic solar power harvesting system. Thedetection of a ground fault and in response the disconnection of thephotovoltaic panel from the output terminal may eliminate the groundfault. The monitored parameter fault detected may be voltage, currentand/or temperature. One or more of the monitored parameters may be outof a previously specified value range, the photovoltaic panel which notbehaving according to specification is disconnected and the outputterminals are bypassed.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 a illustrates a photovoltaic solar power harvesting system,illustrating features of the present invention.

FIG. 1 b shows more details of a circuit and a photovoltaic panel shownin FIG. 1 a , according to an exemplary feature of the presentinvention.

FIGS. 1 c and 1 d show two exemplary switch circuits for a switch shownin FIG. 1 b which are operable by a controller.

FIG. 1 e shows more details of an active bypass circuit according to anexemplary feature of the present invention.

FIG. 1 f shows a timing diagram of operation for the active bypasscircuit shown in FIG. 1 e.

FIG. 1 g shows an example of system level diagram of a controller andmodules which may be implemented in the circuit of FIG. 1 b.

FIG. 2 a shows a method which may be implemented in the circuit of FIG.1 b.

FIG. 2 b shows an exemplary method for a circuit which considers the useof an arc detection module with a theft detection module.

FIG. 3 shows a method for arc detection in a power harvesting systemshown in FIG. 1 a.

DETAILED DESCRIPTION

Reference will now be made in detail to features of the presentinvention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to the like elementsthroughout. The features are described below to explain the presentinvention by referring to the figures.

Before explaining features of the invention in detail, it is to beunderstood that the invention is not limited in its application to thedetails of design and the arrangement of the components set forth in thefollowing description or illustrated in the drawings. The invention iscapable of other features or of being practiced or carried out invarious ways. Also, it is to be understood that the phraseology andterminology employed herein is for the purpose of description and shouldnot be regarded as limiting.

It should be noted, that although the discussion herein relatesprimarily to photovoltaic systems, the present invention may, bynon-limiting example, alternatively be configured using otherdistributed power systems including (but not limited to) wind turbines,hydro turbines, fuel cells, storage systems such as battery,super-conducting flywheel, and capacitors, and mechanical devicesincluding conventional and variable speed diesel engines, Stirlingengines, gas turbines, and micro-turbines.

By way of introduction aspects of the present invention are directed tocircuitry integrated or integrable with a photovoltaic panel to form aphotovoltaic module. The circuitry may include multiple features formonitoring the performance of the photovoltaic panel, detection andelimination of arcs, and/or detection and elimination of ground faultsin the photovoltaic module in or in the vicinity of the photovoltaicmodule or elsewhere in the photovoltaic power harvesting system. Thecircuitry may also include functionality for theft detection andprevention. The circuitry may also include functionality for providingboth a safety mode of operation which features a current limited outputand a normal mode of operation for production of solar power.

According to an exemplary feature of the present invention, the circuitis connected or connectible at the input terminals to a photovoltaicpanel. The output terminals may be connected to form a string ofphotovoltaic modules. Multiple photovoltaic modules may be parallelconnected to form the photovoltaic solar power harvesting system

The term “vicinity” as used herein in the context or arc and/or groundfault detection may refer to another like photovoltaic module connectedin series to form the serial string, another part of the serial stringor another string, e.g. a neighboring photovoltaic string connected inparallel.

The term “current bypass” or “bypass” as used herein refers to alow-resistance direct current connection between the two input terminalsand/or between two output terminals of the circuit to form analternative path for direct current and/or power externally applied tothe terminals. The bypass provides a current path for string current inthe case the photovoltaic panel is disconnected by activation of theswitch.

The term “passive” device as used herein, refers to the “passive” devicenot requiring external power from a source of power to perform a circuitfunction.

The term “active” device as used herein, refers to the “active” devicewhich requires power from an external source of power to perform acircuit function.

The term “switch” as used herein refers to an active semiconductorswitch, e.g. a field effect transistor (FET) in which a controllableand/or variable voltage or current is applied to a control terminal,e.g. gate, of the switch which determines the amount current flowingbetween the poles of the switch, e.g. source and drain of the FET.

The term “activate” a switch as used herein may refer to opening,closing and/or toggling i.e. alternatively opening and closing theswitch.

Reference is also now made to FIG. 1 a of a photovoltaic solar powerharvesting system 10, illustrating aspects of the present invention.Power harvesting system 10 includes multiple photovoltaic panels 101connected respectively to multiple circuits 103. Circuit 103 may behoused in a junction box to provide electrical terminations, mechanicalsupport of bus bars a, b and c (not shown) which may be used as an inputto circuit 103 from a panel 101. Alternatively, circuit 103 may beintegrated with photovoltaic panel 101 without the use of a junctionbox. Circuit 103 may be attachable and/or re-attachable to panel 101 ormay be permanently attachable to panel 101 using for example a thermosetadhesive, e.g. an epoxy adhesive. The electrical outputs of circuits 103may be connected in series to form a series photovoltaic serial string107 through which a string current (I_(string)) may flow. Multiplestrings 107 may be connected in parallel and across an input of a load105. Load 105 may be a direct current (DC) load such as a DC motor, abattery, an input to a DC to DC converter or a DC input to a DC to ACinverter.

A central unit 109 may be operationally connected by control line 114 toand located in the vicinity of load 105. Central unit 109 include atransmitter and/or receiving for transmitting and receiving power linecommunications (PLC) or wireless communications 117 to and from circuits103. Current and/or voltage sensors 119 a, 119 b operatively attached tocentral unit 109 may sense the input of load 105 so as to measure inputvoltage (VT) and input current (IL) to load 105. Central unit 109 mayalso be operatively attached to a network 115, e.g. Internet for thepurposes of remote monitoring or control of system 10. Central unit 109may also serve as to send appropriate control signals to circuits 103based on previously determined operating criteria of power harvestingsystem 10. Alternatively or in addition, a master circuit 103 a in astring 107 may provide independent control within a string 107 and/ormay work in conjunction with central unit 109.

Reference is now made to FIG. 1 b which shows more details of circuit103 and photovoltaic panel 101 shown in FIG. 1 a , according to anexemplary feature. According to the example, photovoltaic panel 101includes two sub-strings 11 of serially connected photovoltaic cellswhich output to bus bars a, b and c which are the input terminals tocircuit 103. Circuit 103 may be housed in a junction box to provideelectrical terminations, mechanical support of bus bars a, b and c andto provide the input terminals to circuit 103. The input of circuit 103includes two bypass diodes 120 a and 120 b with anodes connectedrespectively to bus bars c and b and cathodes connected respectively tobus bars a and b. A transceiver 126 may also be operatively attached tocontroller 122. Transceiver 126 may provide power line communications(PLC) at node Y and/or node X. Transceiver 126 may alternatively providewireless communications. A single pole switch SW1 connects seriallybetween the cathode of diode 120 a and node X. The control of switch SW1is operatively attached to controller 122. Switch SW1 may be opened andclosed by controller 122. A bypass circuit 121 is connected across nodesX and Y. Nodes X and Y provide connection of a circuit 103 into serialstring 107. An alternative implementation of bypass circuit 121 shown inFIG. 1 b , may have bypass diodes 120 a and 120 b replaced by two bypasscircuits 121.

During normal operation of solar power harvesting system 10, panels 101are irradiated by the Sun, panel 101 current (I_(PV)) is substantiallyequal to the string current (I_(string)), switch SW1 is closed andcurrent (I_(B-out)) flowing through output bypass circuit 121 issubstantially zero. The maximum string current (I_(string)) is normallylimited by the worst performing panel 101 in a photovoltaic string 107by virtue of Kirchhoff current law.

In a panel 101, if certain photovoltaic cells in sub-string 11 areshaded, the current passing through the shaded cells may be offered analternative, parallel path through the inactive cells, and the integrityof the shaded cells may be preserved. The purpose of diodes 120 a and120 b is to draw the current away from the shaded or damaged cellsassociated with diodes 120 a and 120 b in respective sub-strings 11.Bypass diodes 120 a and 120 b become forward biased when theirassociated shaded cells in one or more sub-strings 11 become reversebiased. Since the photovoltaic cells in a sub-string 11 and theassociated bypass diodes 120 a and 120 b are in parallel, rather thanforcing current through the shaded photovoltaic cells, the bypass diodes120 a and 120 b bypass the current away from the shaded cells andmaintains the connection to the next sub-string 11.

Controller 122 may be programmed under certain circumstances based onpreviously determined criteria, for instance based on current andvoltage sensed on sensors 124 a-124 d, to open switch SW1, and therebydisconnect panel 101 from serial photovoltaic string 107. Bypass circuit121 may be configured to provide a low impedance path such that theoutput bypass current (I_(B-out)) of bypass circuit 121 is substantiallyequal to string 107 current (I_(string)). Bypass circuit 121 allowsdisconnection of photovoltaic panel 101 from photovoltaic string 107while maintaining current flow and power production from the remainingphotovoltaic panels 101 of photovoltaic string 107.

Reference is now made to FIGS. 1 c and 1 d which show two variant switchcircuits controllable by controller 122 for switch SW1 shown in FIG. 1 b. The first switch circuit switch SWa is a single pole switch orsemiconductor switch, e.g. FET, with a diode connected in parallelacross the single pole switch. Switch SWa may be connected seriallybetween node X and the cathode of diode 120 b with the anode of thediode of switch SWa connected to the cathode of diode 120 b and thecathode of the switch diode to node X. When switch SWa is open circuit,current from panel 101 to node X may flow through the diode of switchSWa and any reverse current from node X may be blocked. A similar seriesarrangement for switch SW1 is shown in FIG. 1 d where switch SWa iswired in series with another switch SWb.

Switch SW1 may alternatively or in addition be connected at the lowvoltage terminal between node Y and the anode of diode 120 a. Analternative arrangement for switch SW1 may have switch SWa connectedserially between node X and the cathode of diode 120 b and to haveanother switch SWb connected serially between node Y and the anode ofdiode 120 a. In this alternative, the diode of switch SWb has an anodeconnected to node Y and a cathode connected to the anode of diode 120 a.In this alternative, when both switches SWa and SWb are open circuit,current from panel 101 to node X may flow through the diode of switchSWa and any reverse current from node X may be blocked. Similarly,current from node Y to panel 101 may flow through the diode of switchSWb and any reverse current from node Y may be blocked.

Reference is now made to FIG. 1 e which shows more details of an activebypass circuit 121 according to an exemplary feature. Bypass circuit 121includes switches SW2 and SW3 (operatively attached to a controller 130)and a charging circuit 141. Switches SW2 and SW3 in the example areimplemented using metal oxide semiconductor field effect transistors(MOSFETs). Alternative solid state switches, e.g. bipolar transistorsmay be used for switches SW2 and SW3. The drain (D) of switch SW2connects to node X. The source (S) of switch SW2 connects to the source(S) of switch SW3. An integral diode of switch SW2 has an anodeconnected to the source (S) of switch SW2 and a cathode connected to thedrain (D) of switch SW2. The drain (D) of switch SW3 connects to node Y.Switch SW3 may have an integral diode with an anode connected to thesource (S) of switch SW3 and a cathode connected to the drain (D) ofswitch SW3. Controller 130 connects to and senses node Z where thesource of switch SW2 connects to the source (S) of switch SW3.Controller 130 connects to and senses node X and also connects to andsenses node Y the drain (D) of switch SW3. Controller 130 also providesthe direct current (DC) voltage (V_(logic)) required by buffer driversB1 and B2. Buffer drivers B1 and B2 ensure sufficient power is availableto turn switches SW2 and SW3 on and off. The outputs of buffer driversB1 and B2 are connected to the gates (G) of switches SW2 and SW3respectively. Buffer drivers B1 and B2 receive their respective logicinputs from controller 130. Charging circuit 141 has an input whichconnects to node Y and to node Z. Connected to node Z is the anode of azener diode Z1. The cathode of zener diode Z1 connects to node Y. Zenerdiode Z1 may be alternatively implemented as a transient voltagesuppression (TVS) diode. A charge storage device, e.g. capacitor C1 hasone end connected to the cathode of diode rectifier DR1 and the otherend of charge storage device C1 connected to node Z. The anode of dioderectifier DR1 connects to node Y. Charge storage C1 device may be acapacitor, a battery or any device known in the art for storingelectrical charge. The end of capacitor C1 connected to the cathode ofdiode rectifier DR1 provides the DC voltage (V_(logic)) to controller130 and buffer drivers B1 and B2.

During the normal operation of power harvesting system 10 during whichpanels 101 are irradiated, the output of a circuit 103 need not bebypassed by bypass circuit 121. Bypass circuit 121 does not bypass byvirtue of switches SW2 and SW3 both being off (open). Switches SW2 andSW3 both being off means substantially no current between respectivedrains and sources of switches SW2 and SW3 because the respective gates(G) of switches SW2 and SW3 are not been driven by buffer drivers B1 andB2.

By virtue of the analog inputs of controller 130 to the source (S) anddrain (D) of switches SW2 and SW3 respectively and the source (S) ofswitch SW3, controller 130 is able to sense if an open circuit or areverse voltage polarity exists across nodes X and Y. The open circuitsensed on nodes X and Y may indicate that switch SW1 is open and/or asub-string 11 is open circuit. The reverse polarity across nodes X and Ymay indicate that a panel 101 is shaded or faulty or that the panel 101is operating as a sink of current rather than as a source of current.

The open circuit and/or the reverse polarity across nodes X and Y maycause bypass circuit 121 to operate in a bypass mode of operation. Thebypass mode of operation of bypass circuit 121 may be when a panel 101is partially shaded. The bypass mode of operation of circuit 121 mayalso be just before the normal operation when it still too dark toobtain a significant power output from panels 101, circuit 121 may haveno power to work.

Reference is now made to FIG. 1 f which shows a timing diagram forcircuit 121 operation. As soon as sufficient light irradiates panels 101and current flows in photovoltaic string 107, zener diode Z1 has voltagedrop VZ1 which charges capacitor C1 so as to provide V_(logic) tocontroller 130. When capacitor C1 is being charged during time T1, thevoltage drop of the output across nodes X and Y is the voltage (VZ1) ofzener Z1 plus the voltage across the integral diode of switch SW2. WhenV_(logic) is sufficient, all the active circuitry in controller 130starts to work which closes switches SW2 and SW3 for a time period T2.Time period T2 may be much greater than time period T1. Switches SW2 andSW3 being closed (during time T2) gives a voltage drop across nodes Xand Y. Therefore, with the longer time period T2 and the voltage dropacross nodes X and Y, overall, less power may be lost by bypass circuit121. Controller 130 continues to work until the voltage (V_(logic)) ofcharge storage device C1 drops below a minimal voltage and once againcharge storage device C1 has voltage drop VZ1 from zener Z1 whichcharges capacitor C1 so as to provide V_(logic). Once sufficient poweris generated from panels 101, controller 130 can get a voltage supplyfrom a panel 101 at nodes X and Y. Controller 130 may also furtherreceive an external enable in order to work in synchronization with allthe other bypass circuits 121 in a photovoltaic string 107.

During the bypass mode, controller 130 is able to sense on nodes X and Yif a panel 101 is functioning again and so controller 130 removes thebypass. The bypass across nodes X and Y is removed by turning switchesSW2 and SW3 off.

Reference now made to FIG. 1 g which shows an example of a system leveldiagram of a controller 122 which may be implemented in a circuit 103.Controller 122 includes a processor 16 which may be operatively attachedto transceiver 126, switch SW1, sensors 124 a-124 d and storage 18.Storage 18 may include software modules and/or additional circuitry mayprovide functionality such as: for monitoring performance of thephotovoltaic panel 160, ground fault detection 166, safety/normal modeoperation 169, arc detection and elimination 162, theft detection andprevention 164. Circuit 103 may be configured to avoid power conversion,e.g. DC to DC conversion during normal power production. Circuit 103 maybe configured to avoid maximum power point tracking of photovoltaicpanel 101. In some configurations, switch SW1 may be a single switch,e.g. FET and therefore extra components, e.g. FET switches may beavoided.

160 Monitoring Performance and Control of Photovoltaic panel 101 andCircuit 103

Monitoring performance of photovoltaic panels has been disclosed by thepresent inventors in US patent publication 2008/0147335. Monitoring mayinclude monitoring input power at the input terminals (bus bars a,b,c)of circuit 103 and/or output power at output terminals nodes X and Y ofcircuit 103 by sensing current and voltage using sensors 124 a-124 d ofcircuit 103. Temperature sensors (not shown) may also be included incircuit 103 for measuring ambient temperature, temperature on thecircuit board of circuit 103 and/or temperature of the photovoltaicpanel 101. Monitoring results may be periodically or randomlytransmitted to central unit 109 by communications over DC lines toinverter 105 or by wireless communication. Based on the monitoringresults, if one or more sensed parameters are found out of ratedspecification, controller 122 may be programmed to activate, e.g. openswitch SW1 and to disconnect photovoltaic panel 101 from photovoltaicstring 107. Bypass circuit 121 autonomously bypasses string currentaround SW1 and photovoltaic panel 101.

DC power cables connecting load 105 to photovoltaic panel 101 and/orcircuits 103 may provide a communication channel between central unit109 and photovoltaic panels 101 As previously disclosed by the presentinventors in co-pending patent application GB1100463.7, lengths ofcables connecting load 105 to panels 101 or circuits 103 may be long andmay contain one or several wire cores. The topography of a distributedpower generation system to a large extent dictates the installation andplacement of cable runs. Physical proximity of wires not having anelectrical association may increase the chances of the wires in thecables being subject to the effects of noise if those wires are to beconsidered for signaling by DC power line communications. Crosstalk is atype of noise which refers to a phenomenon by which a signal transmittedon a cable, circuit or channel of a transmission system creates anundesired effect in another cable, circuit or channel Crosstalk may beusually caused by undesired capacitive, inductive, or conductivecoupling from one cable, circuit or channel, to another. Crosstalk mayalso corrupt the data being transmitted. Known methods of preventing theundesirable effects of crosstalk may be to utilize the shielding ofcables, junction boxes, panels, inverters, loads or using twisted paircables. Additionally, filtering techniques such as matched filters,decoupling capacitors or chokes may be used to prevent the undesirableeffects of crosstalk. However, these ways of preventing the undesirableeffects of crosstalk may be unavailable or impractical in a powergeneration system and/or may be prohibitively expensive in terms ofadditional materials and/or components required.

Within photovoltaic installation 10, a wire at positive potential and awire at negative potential electrically associated therewith may bephysically proximate thereto only at a point of connection to a piece ofequipment. However, elsewhere in photovoltaic field 10, the wires may beseparated and not be within the same cable run. In a photovoltaic powergeneration system, with power line communication over DC cables, it maybe desirable to send a control signal or receive a monitoring signalbetween central unit 109 and circuit 103. Crosstalk may cause the othercircuits 103 in power generation system 10 to inadvertently receive thecontrol signal which is of course undesirable.

A method is disclosed, whereby signaling between a photovoltaic module101/103 and a load 105 provides an association between the photovoltaicmodule 101/103 and the load 105. In an initial mode of operation, aninitial code may be modulated to produce an initial signal. The initialsignal may be transmitted by central unit 109 along DC line from load105 to circuit 103. The initial signal may be received by circuit 103.The operating mode may be then changed to a normal mode of operation,and during the normal mode of operation a control signal may betransmitted central unit 109 along DC line from load 105 to circuit 103.A control code may be demodulated and received from the control signal.The control code may be compared with the initial code producing acomparison. The control command of the control signal may be validatedas a valid control command associated with load 105 with the controlcommand only acted upon when the comparison is a positive comparison.

166 Ground Fault Detection

As previously disclosed by the present inventors in co-pendingapplication GB1020862.7, a device may be adapted for disconnecting atleast one string carrying direct current power in multipleinterconnected strings. Similarly, circuit 103 may include adifferential current sensor adapted to measure a differential current bycomparing respective currents in the positive lines (terminating at nodeX) and negative line (terminating at node Y). The differential currentmay be indicative of a ground fault in circuit 103 and/or photovoltaicpanel 101. If a potential ground fault is detected, then SW1 and/or asimilar switch in the negative line may be activated, e.g. opened.Bypass circuit 121 may autonomously bypass string current around SW1 andphotovoltaic panel 101.

169 Safety/Normal Mode Operation

During normal mode operation of circuit 103, electrical power producedby photovoltaic panel 101 is provided to string 107. Maximum power pointtracking may be provided at the input of load 105 for the interconnectedstrings so that in absence of shading or component failure most or allof photovoltaic panels contribute to the harvested power at or near themaximum power point. In conventional solar power harvesting systems,potential electric shock hazard may exist on the output terminals of thephotovoltaic module 101/103.

Consequently, during installation of a conventional system, photovoltaicpanels may be covered to avoid light absorption by the photovoltaicpanels and to prevent electrocution during installation.

A safety mode of operation may be provided by activating or togglingswitch SW1, which may be a portion of a buck and/or boost converter incircuit 103 attached to a photovoltaic panel 101. Toggling switch SW1 ata known duty cycle may be used to force photovoltaic panel 101 far awayfrom its maximum power point and the power output to string 107 may beforced to be very low avoiding other safety means such as coveringphotovoltaic panels during installation.

During the safety mode of operation, photovoltaic module 101/103 may beconnected or disconnected and while being irradiated by the sun.Therefore, during the routine maintenance or installation of the powerharvesting system 10, controller 122 of circuit 103 may be configured toopen and close switch SW1 to produce a safe working output power onoutput terminals of the circuit 103. The safe working output power maybe according to a predetermined duty cycle of switch SW1 opening andclosing.

During the normal operation of the power harvesting system 10 when powerharvesting system 10 is irradiated, it may be that photovoltaic module101/103 is disconnected from a string 107 as a result of a malfunctionor theft. In the case of theft it may well be desirable that a safeworking output power on output terminals of the circuit 103 is producedso that a thief is not electrocuted for example.

164 Theft Detection

A number methods and/or devices for detection and/or theft prevention ofphotovoltaic panels are disclosed by the present applicant(s) in UnitedStates Patent Application 20100301991.

The use of codes is discussed above as a mechanism to avoid cross talkin monitoring and control signals carried over DC lines to central unit109. Codes may be additionally used as a mechanism for theft detectionand prevention. A first code is written in memory associated with load105 and a second code is stored in the memory 18 located and operativelyattached to circuit 103. The second code may be based on the first codeor the second code may be a copy or a hash of the first code. Thewriting of the first code and/or the storing of the second code may beperformed during installation of the power harvesting system. After thefirst code is read and stored in the first memory, and the second codeis read and stored in memory 18, during the electrical power generation,the first code is compared with the second code or its hash. If thecomparison is correct, (for instance the codes correspond) then powertransfer from circuit 105 to string 107 is allowed, and switch SW1 isclosed. Otherwise, if the codes do not match then switch SW1 is openedby controller 122. If circuit 105 is permanently attached or highlyintegrated with photovoltaic panel 101 then it will be difficult for thethief to benefit from the theft. Other methods for theft detectionand/or protection as disclosed in international applicationPCT/IB2010/052413 may similar be used in conjunction with the presentdisclosure.

162 Arc Detection

Electric arcing can have detrimental effects on electric powerdistribution systems and electronic equipment. Arcing may occur inswitches, circuit breakers, relay contacts, fuses and poor cableterminations. When a circuit is switched off or a bad connection occursin a connector, an arc discharge may form across the contacts of theconnector. An arc discharge is an electrical breakdown of a gas whichproduces an ongoing plasma discharge, resulting from a current flowingthrough a medium such as air which is normally non-conducting. At thebeginning of a disconnection, the separation distance between the twocontacts is very small. As a result, the voltage across the air gapbetween the contacts produces a very large electrical field in terms ofvolts per millimeter. This large electrical field causes the ignition ofan electrical arc between the two sides of the disconnection. If acircuit has enough current and voltage to sustain an arc, the arc cancause damage to equipment such as melting of conductors, destruction ofinsulation, and fire.

FIG. 3 shows a method 301 for arc detection in system 10 shown in FIG. 1a . In step 303 an initial mode of operation for system 10 is initiated.The initial mode may be when system 10 is first installed, when afterinstallation on a daily basis panels 101 are illuminated at dawn orafter a routine maintenance of system 10 where panels 101 may have beenreplaced or cables reconnected etc. The initial mode may also beinitiated at various times during the day and times of the month. Theinitial mode initiated at various times during the day and times of themonth may be performed in respect to the fact that the orientation ofthe sun varies throughout the year. The initial mode may take intoaccount other factors such as temperature, cloud cover or accumulateddust deposition on the surfaces of a panels 101 for example.

In the initial mode, a baseline noise voltage or current may be measured(step 305) for a string 107 or a group of interconnected strings 107 asshown in system 10 and the overall noise voltage or current for systemmeasured at load 105 via sensors 119 a and 119 b. The initial modeinitiated at various times during the day and times of the month may bestored in a look up table in central unit 109 and/or master circuit 103a or in each circuit 103. As a result of the baseline noise voltage orcurrent measured in step 305 a noise voltage or current threshold 309may be set in step 307. Threshold 309 may be an adaptive or a constantvalue which may be measured in frequency range between 10 kilo-Hertz(kHz) to 400 kHz. Once the threshold 309 value has been set for system10, normal operation of system 10 is initiated in step 311. If thethreshold value 309 is exceeded for a predefined time, indicatingpotential arcing, a panel 101 may be disconnected (step 205) from astring 107 using switch SW1 in the circuit 103 associated with the panel101. Otherwise normal operation of system 10 continues in step 311.

Reference is now made to FIG. 2 a which shows a method 251 applicable tosystem 122 shown in FIG. 1 g . In decision 253 a number of malfunctionsmay be detected which including arc detection 162, theft detection 164,ground fault detection 166, or a monitored parameter fault detection. Itis possible in decision 253 to have various combinations of detectiontogether; for example, arc detection 162 along with theft detection 164or arc detection 162 with theft detection 164 and ground fault detection166. A detection of a malfunction may cause switch SW1 to open todisconnect panel 101 from string 107 and the output terminals of circuit103 output may be autonomously bypassed by bypass circuit 121 (step255).

Reference is made to FIG. 2 b which shows an exemplary method 201 forcircuit 103. In decision 203, if arcing is detected in the vicinity of apanel 101, panel 101 may be disconnected from a string 107 by openingswitch SW1 in circuit 103. Panel 101 may be then bypassed using bypass121. In decision 209, methods for arc detection may be applied to verifyif arcing has been eliminated by bypassing circuit 103. If in decisionblock 209, arcing has not been eliminated, panel 101 may be re-connectedin step 211 and another panel 101 may be selected in the string 107 anddisconnected from string 107. Testing for arc elimination continues indecision 209. In decision 209 it may well be that if an arc is noteliminated, a whole string 107 may be disconnected by opening switchesSW1 in string 107 and another string 107 may be checked to see if arcingmay be taking place there instead.

A similar method to that shown in method 201 may also be applied toground fault detection 166.

The indefinite articles “a”, “an” is used herein, such as “a switch”, “amodule” have the meaning of “one or more” that is “one or more switches”or “one or more modules”.

Although selected features of the present invention have been shown anddescribed, it is to be understood the present invention is not limitedto the described features. Instead, it is to be appreciated that changesmay be made to these features without departing from the principles andspirit of the invention, the scope of which is defined by the claims andthe equivalents thereof.

The invention claimed is:
 1. A circuit comprising: a first node, asecond node, and a third node; a first switch connected between thefirst node and the second node; a second switch connected between thesecond node and the third node; a first diode connected between thesecond node and the third node; a charge storage device connected to thesecond node; and a second diode with an anode connected to the thirdnode and a cathode connected to the charge storage device.
 2. Thecircuit of claim 1, wherein the first diode is a zener diode.
 3. Thecircuit of claim 1, wherein the first diode is a transient voltagesuppression (TVS) diode.
 4. The circuit of claim 1, wherein the chargestorage device includes a capacitor.
 5. The circuit of claim 1, furthercomprising a third diode connected between the first node and the secondnode, and a fourth diode connected between the second node and the thirdnode.
 6. The circuit of claim 5, wherein the third diode is an integraldiode of the first switch, and the fourth diode is an integral diode ofthe second switch.
 7. The circuit of claim 1, wherein the first switchand the second switch are metal oxide semiconductor field effecttransistors.
 8. The circuit of claim 1, wherein a source of the firstswitch is connected to a source of the second switch.
 9. The circuit ofclaim 1, further comprising a controller connected to the third node.10. The circuit of claim 9, further comprising a first buffer driver anda second buffer driver, wherein the first buffer driver and the secondbuffer are connected to the controller, and wherein the first bufferdriver is connected to a gate of the first switch, and the second bufferdriver is connected to a gate of the second switch.
 11. The circuit ofclaim 1, further comprising a third switch connected to the first node.12. The circuit of claim 11, wherein the third switch is connectedbetween the first node and a first photovoltaic panel.
 13. The circuitof claim 12, wherein at least one of the first node or the third node isconnected to a series string, wherein the series string includes asecond photovoltaic panel.
 14. A method comprising: connecting a firstswitch between a first node and a second node; connecting a secondswitch between the second node and a third node; connecting a firstdiode between the second node and the third node; connecting a chargestorage device to the second node; and connecting an anode of a seconddiode to the third node and a cathode of the second diode connected tothe charge storage device.
 15. The method of claim 14, wherein the firstdiode is a zener diode or a transient voltage suppression (TVS) diode.16. The method of claim 14, wherein the charge storage device includes acapacitor.
 17. The method of claim 14, further comprising connecting athird diode between the first node and the second node, and connecting afourth diode between the second node and the third node.
 18. The methodof claim 17, wherein the third diode is an integral diode of the firstswitch, and the fourth diode is an integral diode of the second switch.19. The method of claim 14, further comprising connecting a source ofthe first switch to a source of the second switch.
 20. A systemcomprising: a series string of photovoltaic panels; and a circuitconnected to the series string of photovoltaic panels, the circuitcomprising: a first node, a second node, and a third node; a firstswitch connected between the first node and the second node; a secondswitch connected between the second node and the third node; a firstdiode connected between the second node and the third node; a chargestorage device connected to the second node; and a second diode with ananode connected to the third node and a cathode connected to the chargestorage device, wherein at least one of the first node or the third nodeis connected to at least one photovoltaic panel of the series string ofphotovoltaic panels.